PSI - Issue 13

Marc Moonens et al. / Procedia Structural Integrity 13 (2018) 1708–1713 M. Moonens et al. / Structural Integrity Procedia 00 (2018) 000–000

1712

5

Fig. 5: Crack growth curves for various capillary internal pressures

Fig. 4: Crack growth curves for various capillary diameters

Table 1: Crack growth lives, in function of capillary diameter (all capillaries at 2bar absolute pressure)

Without capillary

0.5mm capillary

1mm capillary

2mm capillary

4mm capillary

32800 cycles

32500 cycles (-0.9%)

32200 cycles (-1.8%)

31000 cycles (-5%)

27000 cycles (-17%)

bar (“over-pressurization”), as putting them to an absolute pressure of less than one bar (0.5bar absolute were typically used in the experimental investigations, see Hinderdael et al. [2017]) would result in under-pressurization at ground level, where the open atmosphere is at an absolute pressure of approximately one bar, but not at high altitudes anymore. Therefore, the influence of the presence of the capillary will be assessed in function of two parameters: the capillary diameter and the capillary internal pressure.

3.1. Influence on crack propagation, depending on capillary diameter

Crack growth curves have been computed for di ff erent capillary diameters, namely 0.5mm, 1mm, 2mm and 4mm. For all these simulations, the capillaries are set to an absolute pressure of 2bar. The crack growth curves (crack length vs. cycles) are presented in Figure 4. The crack growth lives, considered to be the number of cycles required to propagate the crack from a 0 till a c , critical crack length at which the maximum stress intensity factor reaches the fracture toughness of aluminum (here, taken as 917MPa √ mm ), are presented in Table 1. From the results, it can be seen that only large capillaries (4mm is about the third of the lug thickness) have a significant influence on the crack growth behavior, as the crack accelerates in the region of the capillary due to the locally reduced section (the crack resumes to its “normal propagation speed” after having passed the capillary). Conversely, small capillaries (0.5mm, 1mm) have almost no influence on the crack growth. Up to now, capillaries having a diameter of 2mm have been used in experimental investigations (see Strantza et al. [2015], Hinderdael et al. [2017]). The present result shows clearly the interest of working with even smaller capillaries, which is actually the direction the research around eSHM was aiming for (hence the name “capillaries”, implicitly referring to very small diameters) due to a reduced influence on crack initiation. It is worth noting that the manufacturing of components with embedded ultra-small capillaries (0.25 - 0.5mm diameter) will be rendered possible thanks to the research around hybrid manufacturing (additive manufacturing in combination with micro-milling). Post-processing also reveals that the capillary diameter has no influence on the propagation path followed by the crack.

3.2. Influence on crack propagation, depending on capillary internal pressure

A similar study has been conducted, this time with the capillary internal pressure as parameter (all the capillaries having then 2mm as diameter, since, as here above mentioned, this has been up to now the “standard value” in experimental works). The objective is then to determine whether the over-pressurization of the capillary, required for proper functioning of the eSHM system (see above), would lead to some crack retardation or not, and should it be so, to